well maybe the pes is sucking up the excess?
Yes — PES membranes do adsorb copper ions , and this is a well-documented problem in analytical work.
A 2023 study in ACS Omega (“Improving Membrane Filtration for Copper Speciation”) specifically investigated this and confirmed that PES membranes retain Cu²⁺ via Coulombic interaction between the copper ions and the negatively charged sulfonic acid (–SO₃⁻) groups inherent to the polyethersulfone polymer. Key findings:
- At low concentrations (e.g., ~0.2 mg/L Cu), retention is significant and analytically problematic.
- At higher concentrations (~4 mg/L), only a negligible fraction is retained.
- The estimated ion-exchange capacity of the membrane was 0.24–0.66 µequiv/g.
Mitigation: Pretreating the PES membrane with a divalent cation salt before use substantially reduces copper loss. In that study, 0.1 M CaCl₂ was the most effective pretreatment, recovering >93% of Cu even at 0.2 mg/L. BaCl₂ was also effective in the environmentally relevant range.
Practical takeaway: If you’re filtering a dilute copper solution through a 0.22 µm PES syringe filter and need quantitative recovery, expect some loss and consider either (a) pre-conditioning the filter with a CaCl₂ or BaCl₂ solution, or (b) switching to a membrane material without sulfonate groups (e.g., PTFE or nylon) if your matrix allows.
Looks to be the reason why the filtering works.
and yes, I asked afterwards if GHK-Cu would bind:
Short answer: significantly less than free Cu²⁺, and in practice negligible for most use cases.